(1) Field of the Invention
The subject invention relates to a method and system for monitoring the condition of a vehicle wheel mounted tire such as the pressure or temperature within the tire.
(2) Description of the Prior Art
For some time there has been interest in monitoring the air pressure in a pneumatic tire. Because an important factor governing the useful service life of a pneumatic tire is correct inflation pressure, there has been significant interest with truck tires because of their relatively high replacement cost. There is now, however, also significant interest in monitoring the tire inflation pressures on passenger cars because correct inflation pressures significantly improve fuel economy by decreasing tire rolling resistance.
Many systems have been proposed to continuously monitor tire pressure on a vehicle but none have been widely adopted because of difficulties with either cost, complexity, reliability or a combination of these factors.
The prior systems have either been mechanical or electrical with the mechanical systems having been virtually eliminated from consideration. Electrical systems have included a battery powered wheel mounted radio transmitter or a passive circuit, energy absorbing type or an induction or transformer type. The battery type is such that the system draws power from the battery only when a decrease in pressure or rise in temperature occurs to power the transmitter. These systems have not been widely accepted because they are not fail-safe in that, if a malfunction occurs in the system, there is no indication whatsoever. In a fail-safe system, the monitor must be active and self-checking until a fault is sensed such as a decrease in the tire pressure below a predetermined pressure. If a battery were to be utilized in such a system it would have to be continually recharged because the system would be providing a signal at all times except when a fault is sensed.
Passive circuits which have been utilized to sense tire pressure have not had the desired sensitivity and have inherent coupling problems. Furthermore, coupling to the antenna in such systems is a function of the rotational position of the wheel, thereby allowing only intermittent sensing. Reliable pressure sensing on a nonmoving vehicle is, therefore, impossible.